1. Field of the Invention
This invention relates to an integrated optical chip mounted on a substrate. In particular, the invention relates to thin integrated optical chip structures capable of operating at high frequencies in excess of 10 GHz.
2. Technical Background
The term “integrated optics” refers in general to a class of devices for guiding and controlling light in thin film layers or in narrow waveguiding channels formed in suitable dielectric materials (crystal or glass). Integrated optical devices include optical waveguides, switches, polarisers, amplitude and phase modulators, and other functional devices. In system applications, several functional units may be combined (“integrated”) on a single crystalline or glass substrate (an “optical chip”), and the devices may be connected to sources, detectors or other optical devices, usually by means of optical fibre.
As the demand for telecommunications services and bandwidth has boomed, the need for, and advantages of, external modulation in fibre-optic transmission systems has been firmly established. Lithium niobate is today one of the most important dielectric materials in the field of integrated optics, both for research and for technological applications. This importance is due to the strong correlation between the optical properties of the crystal, its refractive index, and the application of electric fields (electro-optic effect) and sound waves (acousto-optic effect). Lithium niobate external modulators provide both the required bandwidth and a means for minimizing the effects of dispersion that limit system performance.
In order to improve the bandwidth, interest has increased in travelling wave electrode configurations, which achieve a close phase match between the optical and microwave signals. In lithium niobate Mach Zehnder interferometers, this has been achieved both by the use of a shielding plane and by the use of thick electrode structures. These devices have the potential for very broadband operation, but they are limited particularly by the electrical loss of the electrodes. One of the principal high frequency electrical loss mechanisms is attributed to a transverse resonance in the substrate of the chip. This effect has been observed with different electrode configurations and has been attributed to a coupling between the fundamental coplanar waveguide mode and a substrate mode—see “Electrical Loss Mechanisms in Travelling Wave LiNbO3 Optical Modulators” by Gopalakrishnan, G. K.; Burns, W. K.; Bulmer, C. H.; Electronics Letters, 1992. The authors demonstrate by simulation and experiments that the frequency at which mode coupling begins varies with substrate thickness. In order to avoid mode coupling at frequencies of up to 10 GHz, a substrate thickness of 1.0 mm is satisfactory. However, in order to avoid mode coupling at frequencies greater than 40 GHz, a substrate thickness of less than 0.25 mm is necessary.
Because of their fragility, the manufacturing processes used in the production of optical modulators using thin substrates becomes critical in all phases, including chip separation, testing and handling during assembly processes. Moreover, test specifications require devices to be able to withstand shocks of 500 g (4900 m/s2). Using lithium niobate chips with a substrate thickness of 0.5 mm, the Applicant has found that around 80% are damaged during assembly, with the remainder being destroyed during shock testing. Telcordia specifications also require devices to be subject to thermal shocks and cycles. Lithium niobate exhibits anisotropic thermal expansion properties: that is to say the dimensional changes in the material associated with temperature variations are different in each direction within the crystal. Typical values of coefficients of thermal expansion (CTE) for an X-cut chip are 15×10−6/° C. and 7×10−6/° C. in orthogonal directions in the same plane. Conventionally, stainless steel (CTE=18×10−6/° C.) and an Fe—Ni—Co alloy known as Kovar™ (a trademark of Westinghouse Electric Corporation) (CTE=5×10−6/° C.) have been used as substrates for mounting lithium niobate chips because their CTE values provide a close match to the CTE in selected directions of the lithium niobate. Although temperature induced stresses due to the mismatch of CTE in other directions are sustainable with LiNbO3 chips of a thickness of around 1.0 mm, such stresses can lead to failure during thermal tests of the attachment between thinner lithium niobate chips and the substrate or to breakage of the brittle crystalline chip.
U.S. Pat. No. 4,750,800 describes a mounting structure for an integrated optical device chip wherein the mounting structure has similar thermal expansion properties to those of the chip in order to minimize stress transmission to the chip due to temperature fluctuations. Each individual chip is mounted on the substrate having a thickness at least ten times greater than the chip thickness.
A further problem of chip design arises due to the pyroelectric effect, in which a charge differential develops in an X-cut chip across the face of a chip due to thermal variation. Such a charge differential can lead to an electrical discharge which can cause errors and distortion to signals processed by the chip. The electrical field that produces the voltage differences across the surface of the chip is caused by any change in the bulk temperature of the chip. A temperature gradient across the chip is not required to produce a voltage difference due to the pyroelectric effect. This spontaneous electrical polarization directed only along the z-axis of the crystal is produced whenever the temperature of the chip is changed from one value to another. The relation between small temperature variations and polarization is linear.
U.S. Pat. No. 6,044,184 describes an integrated optical chip in which a surface is wholly or partially coated with a conductive coating to prevent a charge differential from developing across the surface. Conductive security dabs are used to increase reliability of the conductive paths at the edges of the surface joining the +and −Z faces of the chip.
There remains a need to provide a thin crystalline integrated optical chip which is robust and capable of operating at frequencies in excess of 10 GHz.